1. Field of the Invention
The present invention relates to a power supply unit and a power supply system having a secondary cell or the like to/from which power can be input/output. More particularly, the present invention relates to a power supply unit and a power supply system realizing efficient parallel driving of secondary cells or the like.
2. Description of the Prior Art
Hitherto, as a power supply to/from which power can be input/output, a secondary cell (storage battery) and a capacitor are used. As secondary cells, there are known a battery pack in which a plurality of secondary cells are connected in series with one another and a battery pack in which a plurality of secondary cells are connected in parallel with each other. There is also generally known a method of increasing electric energy which can be continuously output by connecting a plurality of capacitors in parallel with one another.
In a power supply system constructed by connecting a plurality of converters in parallel, there are known a method of calculating an average value of output currents of the plurality of converters and adjusting output current of each of the converters to the calculated average output current and a method of detecting the maximum output current from output currents of the plurality of converters and adjusting output current of each of the converters to the detected maximum output current. Such methods are disclosed in, for example, Japanese Patent No. 2833460 and “Parallel Drive Control of DC-DC Converters” (The Institute of Electronics, Information and Communication Engineers, issued in November, 1992, IEICE Technical Report, PE92-47, pp. 23-29).
On the other hand, in recent years, a fuel cell using hydrogen as a material is being actively developed. FIG. 9 is a diagram showing output characteristics of a fuel cell. A solid line 300 indicates an output characteristic of a fuel cell in the case where the horizontal axis denotes output current and the vertical axis indicates output voltage. As shown by the solid line 300, the fuel cell has the characteristic that as the output of current increases, the output voltage decreases. As understood from a solid line 301 indicating the output characteristic of the fuel cell in the case where the horizontal axis denotes output current and the vertical axis indicates output power, as the output current increases from 0 ampere, the output power of the fuel cell increases. The output power becomes the maximum at current Ij. When the output current is increased more than the current Ij, the output power decreases. When the current exceeding the current Ij is output, the fuel cell may be destroyed, so that it is necessary to control to always use the fuel cell at the current Ij or less.
For example, in the case of using, as a power supply, a battery pack in which “k” (“k”: an integer of 2 or more) secondary cells are connected in parallel with one another, as compared with the case of using a single secondary cell as a power supply, ideally, constant power can be continuously supplied to a load for time which is long by “k” times (that is, the life of the power supply increases by “k” times).
In the battery pack in which “k” secondary cells are connected in parallel with one another, however, even if the capacities of the “k” secondary cells are the same, current flowing in the secondary cells varies due to variations in the internal resistance of the secondary cells. As a result, the powers generated at both ends (both poles) of the secondary cell (the product between voltage across the ends (both poles) of the secondary cell and current flowing in the secondary cell) also varies. Consequently, the time points at which the residual quantities of the secondary cells become zero and the time points at which the secondary cells are fully charged are different from one another. Therefore, actually, it does not mean that by using the battery pack, the life of the power supply increases by “k” times (actually, the life increases less than “k” times).
This phenomenon similarly occurs also in the case of using capacitors in place of secondary cells. Further, there is another problem in that a secondary cell having low internal resistance is over-discharged or over-charged and the characteristics of the secondary cell deteriorate.
Also in the case where a power supply is constructed by connecting “k” fuel cells in parallel with one another, current flowing in the fuel cells varies due to variations of output impedances of the fuel cells. In this case, current is output preferentially from a fuel cell having low output impedance, so that the output current of the fuel cell having low output impedance exceeds the current Ij (FIG. 9) and a problem occurs such that the fuel cell is destroyed.